Binary Droplets Walking With Their Feet.

We shed new light on paramount but so far highly elusive morphological features of perfectly wetting binary sessile droplets evaporating into air with a more volatile component possessing higher surface tension (water + propylene glycol). By both interferometric measurements of the detailed shapes of the drops and theoretical developments, we show that there is a narrow high-curvature zone due to the solutal Maranoni effect localized near the edge (foot) of the droplet that dictates the overall droplet shape and behavior by means of induced apparent contact angles. The phenomenon is studied as a function of the mass fraction of the components in the droplet and the ambient relative humidity. We devise a local model, based on the lubrication approximation and Marangoni-flow-related Taylor dispersion, that enables to predict the apparent contact angles without any fitting parameters or invoking any microphysics. Motion dynamics of such droplets in a humidity gradient is also studied.